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J. Anim. Sci. 2005. 83:1423-1434
© 2005 American Society of Animal Science

ANIMAL PRODUCTION

A model of fescue toxicosis: Responses of rats to intake of endophyte-infected tall fescue1,2

D. E. Spiers*,3, P. A. Eichen* and G. E. Rottinghaus{dagger}

* Animal Sciences Unit, and and {dagger} Veterinary Medical Diagnostic Laboratory, University of Missouri, Columbia 65211


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 Implications
 Literature Cited
 
A study was conducted to develop a model for fescue toxicosis using rats fed a diet containing endophyte-infected tall fescue seed (E+). Rats implanted with telemetric transmitters to continuously monitor core body temperature (Tc) and activity were housed at thermoneutrality (21°C) and were fed a diet containing endophyte-free fescue seed (E–). After 2 wk, they were assigned to either E+ or E– diets and initially maintained at thermoneutrality (preheat) for 8 d. They were then exposed to heat stress (31°C) for 22 d, followed by 1 wk of recovery at thermoneutrality (post-heat). Body weight and feed intake were measured daily. Rats receiving the E+ diet showed decreased feed intake (P = 0.001) and weight gains (P = 0.003) during the preheat period. The decrease in Tc from the pre-treatment level was greater in E+ than in E– rats during the preheat (P = 0.001) and postheat (P = 0.001) periods. With heat stress, both groups showed parallel decreases in feed intake. The increase in Tc from pre-heat to heat conditions was greater in E+ vs. E– rats (P = 0.001). Activity level was lower in E+ than in E–rats during heat stress (P = 0.009) and postheat (P = 0.037) periods. These results show that the rat model for fescue toxicosis is extremely useful because many of the observed responses to E+ diet are similar to those noted for cattle, and additional variables that are difficult to measure in cattle, such as activity, can be easily evaluated.

Key Words: Heat • Neotyphodium • Rat • Tall Fescue • Temperature


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 Implications
 Literature Cited
 
Cattle grazing endophyte-infected tall fescue often experience health and production problems that are collectively referred to as fescue toxicosis. Summer syndrome (i.e., a subset of fescue toxicosis) occurs during heat stress and is characterized by hyperthermia, with an accompanying decrease in feed intake and growth. Annual economic losses to the beef industry from decreased conception rate and weaning weight associated with fescue toxicosis have been estimated at $600 million (Hoveland, 1993Go). Additional losses due to altered thermoregulatory ability, decreased feed intake, and reduced ADG in older animals further increases the adverse economic effect of fescue toxicosis. Rats receiving a single injection of ergovaline, a primary toxin found in endophyte-infected tall fescue (E+), at thermoneutrality exhibited decreased rectal temperature and increased tail temperature (Zhang et al., 1994Go), suggesting increased heat loss. A similar treatment during short-term heat stress increased rectal temperature and decreased tail temperature (Spiers et al., 1995Go), indicating decreased heat loss. Neal and Schmidt (1985)Go fed E+ diets to rats for 15 d and observed decreased feed intake, growth rate, and lowered internal body temperature, but did not subject the animals to heat stress. More recently, Roberts et al. (2002)Go found rats fed an E+ diet exhibited decreased feed intake in both thermoneutral and heat stress environments. A more complete rodent model of fescue toxicosis should include long-term change in internal body temperature as a key indicator of fescue toxicosis. The present study was conducted to include this important indicator along with general behavior before, during, and after exposure to heat stress. Development of this model for fescue toxicosis would provide a rapid, economical determination of multiple effects of fescue toxicosis, and allow for preliminary evaluation of potential treatments.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 Implications
 Literature Cited
 
Animals

Male Sprague-Dawley rats (n = 12; 248.77 ± 5.24 g BW; 40 d of age) were assigned randomly to E+ (n = 6) or endophyte-free (E–; n = 6) treatment groups. Animals were initially housed in individual cages at thermoneutrality (21°C, 55% relative humidity). Lights were on from 0700 to 1900 daily throughout the study. Rats were provided ad libitum access to water and feed throughout the study. A telemetric, temperature transmitter (Mini-Mitter, Inc., Bend, OR; http://www.minimitter.com/Products/Vitalview/Series3000.html) was implanted into the peritoneal cavity of each rat 2 wk before testing, and used for repeated measurement of core body temperature (Tc) and gross motor activity. The transmitter system consisted of a grid across the bottom of the cage that responded to horizontal, not vertical, movement. Movement of the transmitter with the rat resulted in small changes in the emitted signal that were detected by the TR-3000 receiver and sent to a computer for storage. The output used for analysis was activity counts over a specific period of time. There was no identification or separation of specific activities, such as drinking and feeding behaviors. Body weight and feed intake were measured daily to 0.1 g, and Tc and activity levels were recorded at 20-min intervals.

Treatment Diet

Diets contained either E– tall fescue seed (Miller Seed Co., Clinton, MO) or E+ tall fescue seed (Seed Research of Oregon, Inc., Corvallis, OR), which were ground in a Thomas Wiley laboratory mill (model 4, Arthur H. Thomas Co., Philadelphia, PA) to first pass a 2- and then a 1-mm screen. All seeds were tested for ergovaline content according to the protocol described by Hill et al. (1993)Go and the HPLC procedure presented by Rottinghaus et al. (1991)Go. Analysis of the E– seed showed an ergovaline concentration of less than 50 ppb on a DM basis. In contrast, the E+ seed was determined to contain 5,000 ppb ergovaline on a DM basis. Seeds also were analyzed for Claviceps purpurea ergo-peptine alkaloids using the procedure of Rottinghaus et al. (1993)Go, with a detection limit of 50 ppb for the total concentration of the five ergopeptine alkaloids. Diets (Table 1Go) were stored at 4°C, and the E+ diet was formulated to contain 2.06 µg of ergovaline/g of diet and deliver approximately 165 µg of ergovaline·kg BW–1·d–1 in the thermoneutral environment. It was certain that this dose would produce traditional symptoms of fescue toxicosis because it was 16 times greater than the dose used to produce similar symptoms in cattle (i.e., 10 µg of ergovaline·kg BW–1·d–1; Spiers et al., 2004Go).


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  		Table 1. Composition of rat diet (as-fed basis)a
 
Treatment Period

Rats were fed standard, ground rat chow (Formulab diet 5008, PMI Nutrition Int., Brentwood, MO) during recovery from surgery. After a 2-wk recovery (d –14 to –1), rats were assigned randomly to either control (E–) or treatment (E+) diets, and maintained initially at thermoneutrality for 8 d (preheat; d 1 through 8). Ambient temperature was then increased to heat stress (31°C) level and held there for 22 d (d 9 through 30), followed by 1 wk of recovery at thermoneutrality (postheat; d 31 through 37).

Statistical Analyses

Data was analyzed using two-way repeated-measures ANOVA procedure and standard least squares model fit (JMP; SAS Inst., Inc., Cary, NC). Components of the statistical model included endophyte treatment, time, and treatment x time interactions. When ANOVA revealed a significant difference in least squares means, a Tukey-Kramer multiple comparisons test (Steel and Torrie, 1980Go) was performed at {alpha} = 0.05 to determine both between- and within-treatment effects. All data are reported as least squares means ± SE, and the significance level was preset at P < 0.05 for all analyses.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 Implications
 Literature Cited
 
Feed Intake

Feed intake during the pretreatment period averaged 22.42 g/d (as-fed basis), with no significant difference between treatment groups (Table 2Go). The E+ diet decreased feed intake below the E– level during both the preheat and postheat thermoneutral periods (Table 2Go). Likewise, change in feed intake during treatment, determined from d –1, was different between groups (Table 2Go; Figure 1Go). The initial decrease in feed intake of the E+ group occurred within the first 24 h, with a reduction of 11.92 g/d (P < 0.05; i.e., 52%) below pretreatment level (Figure 1Go). In the present study, there was a partial recovery of 4.7 g/d in feed intake from the d 1 level to d 5, 6, and 8 (Figure 1Go; P < 0.05), suggesting adaptation to the diet and the induced conditions.


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  		Table 2. Average daily values for feed intake (as-fed basis) over the four periods of study and comparison of effects
 


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  		Figure 1. Average daily BW and ADFI (as-fed basis) change from d –1 during the entire treatment period, including preheat (21°C), heat stress (31°C), and postheat (21°C) exposures. Response of animals on the endophyte-infected diet is indicated by E+ (- - -), and on the endophyte-free diet by E– (—). Vertical lines on the first and last values indicate ± 1 SE. Values in boxes indicate treatment and treatment x day effects for each exposure.

 
Exposure to heat stress resulted in a decrease in average daily feed intake for both treatment groups (Figure 1Go). Although the actual drop from preheat to heat stress levels was greater for E– than for E+ groups, the change from pretreatment was much greater for the E+ group (Table 2Go). Feed intake for the E+ animals decreased from d 9 to 13 (P < 0.05) and remained at this level until the end of heat stress (d 30; Figure 1Go). In contrast, feed intake by E– rats decreased more rapidly as evidenced by a decrease (P < 0.05) from d 9 to 11. Differences in ADFI between E+ and E– groups occurred on d 9, 11, 17, 18, 22, 28, 29, and 30 (P < 0.05). The E– group began to recover from heat-induced depression of feed intake during the latter portion of heat stress as seen by the group differences at 28, 29, and 30 d (Figure 1Go).

Removal from heat stress and return to thermoneutrality (postheat) resulted in a parallel recovery of feed intake for both treatment groups and a return to preheat level of intake (Figure 1Go; Table 2Go). Differences between E+ and E– groups remained significant for the entire postheat period (P < 0.001), with differences between the treatment groups on d 32 through 37. Rate of increase in feed intake during this 6-d period was 0.49 and 0.78 g/d for E– and E+ groups, respectively.

Body Weight and Growth Rate

Daily body weight of E+ and E– groups did not differ before treatment (P > 0.10). During the preheat period, daily BW tended to be lower in E+ vs. E– rats. The first indications of significant differences in BW from d –1 value appeared on d 3 and 7 (Figure 1Go) for E–and E+ rats (P < 0.05), respectively, indicating a decrease in growth of E+ animals. In support of this finding, ADG was reduced in E+ rats during this period (1.44 and 6.81 ± 0.94 g for E+ and E–, respectively; P < 0.001). These results were expected given the decrease in feed intake with the E+ diet.

Exposure to 31°C maintained BW of E+ rats below that of E– rats (P = 0.001; Figure 1Go). Heat stress alone produced a 61% decrease in ADG below thermoneutral level for the E– group. The decrease for the E+ group was much less at 35%; however, this was due to the fact that ADG was already reduced significantly during the preheat period. These decreases were expected given the noted reduction in feed intake during heat stress.

Return of rats to thermoneutrality (21°C; postheat) resulted in recovery of growth for both treatment groups that coincided with return of feed intake (Figure 1Go). Average daily BW for E+ rats remained below the E– level (P = 0.001) due to the initial depression during the preheat period. The percentage increase in ADG from heat stress to postheat periods was 109 and 348% for E– and E+ groups, respectively. The difference in ADG between preheat and postheat periods was –18 and +192% for E– and E+ groups, respectively. These results suggest there is partial recovery from the growth depressive effects of ergot alkaloids over time that is similar to the recovery in feed intake.

Core Temperature Response

Average core body temperature of rats in the present study ranged from 37.48 to 37.70°C (Table 3Go) before treatment. These values were within the range of colonic temperature reported by Gordon (1993)Go for adult rats maintained at air temperatures of 20 to 24°C. A statistically significant difference (0.22°C; P = 0.043) in Tc between E+ and E– rats was noted before treatment. Because of this pretreatment difference, change from pretreatment level was included in the analyses. A prominent circadian rhythm for Tc was noted during the pretreatment period (Figure 2Go), using combined values from both groups. Core temperature before 0630 was higher (P < 0.05) than that from 0730 to 1830. Likewise, Tc during the 0630 to 1830 period was significantly lower (P < 0.05) than the values after this time. The abrupt decrease and increase in Tc coincided with lights on and off, respectively, in the environmental chamber. Other researchers have reported a similar circadian rhythm for rat internal body temperature (Scales and Kluger, 1987Go; Gordon, 1993Go).


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  		Table 3. Average daily values for core body temperature over the four periods of study and comparison of effects
 


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  		Figure 2. Average daily core temperature and activity over 24 h during the pretreatment period (i.e., d –1 to –5) and the last full day of treatment at thermoneutrality before exposure to heat stress (i.e., d 8). Response of animals on the endophyte-infected diet is indicated by E+ (- - -), and on the endophyte-free diet by E– (—). Vertical lines on the first and last values indicate ± 1 SE.

 
Evaluation of the preheat period identified no significant treatment effect on Tc (P > 0.10), but a treatment x day interaction (P < 0.001; Table 3Go) was detected. In addition, there was a statistically significant effect of treatment on the change in Tc from the average pretreatment level on d –5 to –1 (P = 0.001), with the E+ group being –0.24°C below that of the E– group (Table 3Go). There was a time-related change in Tc within the preheat period, with the value decreasing (P < 0.05) for E+ rats from d 3 to d 5, 6, and 7. In contrast, there was no change in Tc of E– rats during this period. Significant differences between treatment groups occurred on d 5, 6, and 7 (P < 0.05).

The noted shift in thermal status of the treatment groups during the preheat period was not due to a difference in daily maximum Tc (P > 0.10) but to a shift in minimum daily value (P < 0.001). The daily decrease in combined Tc for both groups occurred mainly from 0630 to 0730 (P < 0.05), and the daily increase was from 1830 to 1930 (P < 0.05). This was identical to the rhythm seen during the pretreatment period. The separation in minimum values is readily observed using a plot of circadian rhythms (i.e., Tc vs. time of day) for both treatment groups on the last day at thermoneutrality (Figure 2Go). On this day, Tc dropped to a lower level in E+ rats compared with E– rats from 0830 to 1230. Although statistically insignificant, the Tc of E+ rats was 0.3 to 0.5°C below that of E– rats during this period. Core temperature in E+ rats increased significantly from 1230 to 1330 (P < 0.05), with a similar increase for E– rats not occurring until 2030.

Exposure to heat stress resulted in a progressive increase in daily mean Tc of both groups over the first 5 d, with a gradual decrease over the remaining 16 d of the heat-stress period (Figure 3Go). There was a day x treatment interaction during this period (P < 0.001; Table 3Go), but the only significant day difference in treatment mean values occurred on d 14, when Tc peaked for both groups (Figure 3Go). The lack of treatment differences was due to the large standard error (0.10°C) for Tc during this period. Larger group differences were noted by determining the heat stress days that were significantly different from d 9 (i.e., initial increase in air temperature). This was possible with the use of the Tukey-Kramer test. The E– group exhibited no significant change in Tc from the d-9 level at any time during the heat stress period. In contrast, Tc for the E+ group was significantly higher than the level on d 9 from d 11 to 28. Change in Tc from pretreatment level exhibited a similar response, with no significant change for the E– group but a significant increase in Tc for the E+ group over the same time period.



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  		Figure 3. Average daily core temperature and activity over 24 h during the entire treatment period that includes preheat (21°C), heat stress (31°C), and postheat (21°C) exposures. Response of animals on the endophyte-infected diet is indicated by E+ (- - -), and on the endophyte-free diet by E– (—). Vertical lines on the first and last values indicate ± 1 SE. Values in boxes indicate treatment and treatment x day effects for each exposure.

 
Circadian change in Tc on d 14 (i.e., peak hyperthermia during heat stress) was different from that seen before heat stress. Both treatment groups exhibited a continuous decrease in Tc from approximately 0 to 1300, with no abrupt decrease in Tc when lights were turned on at 0700. During this descent, Tc for the E+ group was always higher than for the E– group by approximately 0.5°C. Analysis of d-14 values alone showed that Tc was significantly higher (P = 0.0116) in E+ rats (38.4°C) compared with E– rats (37.8°C). In contrast, there was little treatment difference on the same day during the rise in Tc (i.e., 1300 > 2300) under constant heat stress conditions. These circadian differences in Tc from the preheat values on d 8 had disappeared by d 30, suggesting partial adaptation to heat stress conditions.

Core temperatures for E+ and E– groups during the recovery period (postheat) are shown in Figure 3Go. Although there was no difference between absolute Tc for the treatment groups, there was a significant difference when change in Tc from pretreatment level was compared (Table 3Go). This difference was similar to the preheat level noted before heat stress, with E+ rats exhibiting a greater reduction below pretreatment level (–0.29 vs. 0.11°C) than for E– rats. A comparison of change in Tc from the first day of postheat (i.e., d 31) showed no differences for E– rats. In contrast, E+ rats showed a significant decrease (P < 0.05) in Tc from d 31 to d 32 and 35–37.

Analysis of Tc for the first full day at postheat (d 32) showed a significant difference between treatment groups (P = 0.048), with the E+ group averaging below the E– group for the entire day. The major differences between groups occurred between 0 and 1100. After this time, there was overlap between Tc for the treatment groups. As noted for the preheat period, the significant shifts in Tc occurred between 0630 and 0730, and between 1230 and 1930. By the last day of the study (d 37), there were no significant differences in Tc for the treatment groups as a function of time of day. There still was a tendency, however, for Tc of E+ rats to be below E– rats.

Activity

There have been few studies under controlled conditions to identify the effect of fescue toxicosis on general behavior. Analysis of activity before treatment in the present study (Figure 3Go) showed a pattern for animals in both treatment groups that corresponded to the Tc pattern. In general, activity decreased from 0630 to 0730 (lights on; P < 0.05) and increased from 1830 to 1930 (lights off; P < 0.05). As expected, there were no differences between treatment groups during this period (Table 4Go).


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  		Table 4. Average daily values for activity over the four periods of study and comparison of effects
 
A breakdown of the analysis across periods (i.e., preheat, heat stress, and postheat) showed distinct differences for each phase of the study (Table 4Go). Although no significant treatment or treatment x day differences were detected during the preheat period, there was a trend toward a decrease in activity after d 4 (Figure 3Go), and exposure of rats to heat stress resulted in a decrease in activity for rats in both treatment groups. Percent reductions below pretreatment level were 8.8 and 29.3 for E– and E+ groups, respectively (Table 4Go), indicating that fescue toxicosis results in an even greater decrease in activity during heat stress. Although daily activity of E– rats did not change significantly during heat stress from the d 9 to 10 value, E+ rats had decreased activity (P < 0.05) from d 10 to 12, 18 to 23, 27, and 30 (Figure 3Go). Both daily activity and change in daily activity from pretreatment level during heat stress exhibited treatment (P = 0.01) and treatment x day effects (P = 0.02 to 0.04; Table 4Go). All data indicated that fescue toxicosis resulted in a reduction in activity. These differences in activity remained in effect even on the last full day of heat stress (d 30), with E+ rats exhibiting a lower level of activity than those on the E– diet (31.8 vs. 43.8 counts/h; P = 0.004). The return of ambient temperature to thermoneutrality (postheat) resulted in an abrupt rebound in activity followed by a slow decline from d 32 to 38 (Figure 3Go). As noted during heat stress, there was a significant treatment effect on daily activity (P = 0.037), with the E+ group remaining below the E–group (Table 4Go). Although there was no significant treatment effect on change from pretreatment level, there was a trend toward a 20% decrease in activity of E+ animals and only a 3% decrease for E– animals. Analysis of hourly activity differences for early (d 32) and late (d 37) recoveries showed no significant treatment effects, although activity of E+ animals was generally below that of E– animals.


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
Implications
 Literature Cited
 
Rats in the present study exhibited general symptoms that are characteristic of fescue toxicosis. The dietary daily dose of ergovaline used in this study was more than 16 times the amount required to produce similar symptoms of fescue toxicosis in cattle. Several explanations for this species difference have recently been presented by Spiers et al. (2004)Go. The metabolic size of cattle is more than six times less than that of the rat (Kleiber, 1961Go). As a result of this difference, cattle would be expected to metabolize and eliminate E+ toxins at a much slower rate. This would require a higher dose in rats to produce a response that is identical to that in cattle. In addition, fescue seed may remain in the ruminant stomach for a longer period to provide a greater breakdown and extraction of toxins than in the nonruminant, rat stomach. Again, the result would be a requirement for a lower effective dose of ergovaline in cattle than in rats.

Some of the symptoms of fescue toxicosis measured in the present study were independent of ambient temperature and level of thermal stress (Figure 1Go). Affected endpoints included feed intake, BW, and activity. In each case, the values for E+ rats were below E– rats. The general effect of E+ treatment on intake and growth agree with an earlier study using a similar experimental model (Roberts et al., 2002Go). These results verify that the rat model is very sensitive to toxins found in E+. More specific information regarding time-related responses to thermoneutral and heat stress conditions that allow for comparison with domestic animal species can be derived from analysis of responses to treatment during each stage of the present study.

Preheat

Feed intake of rats in the present study at thermoneutrality decreased by more than 50% within the first 24 h on the E+ diet (Figure 1Go). Although it was not measured in this study, it is likely that intake was highest during the active night period, as reported by others (Siegel, 1961Go; Zucker, 1971Go), and decreased to low levels during the day. This decrease was very similar to that reported by Roberts et al. (2002)Go for rats fed E+ at 150 µg of ergovaline·kg BW–1·d–1 (64% of control intake). Likewise, Neal and Schmidt (1985)Go noted that rats fed an E+ diet, at 40% seed, decreased feed intake to 54% of control level. This reduction was similar to many cattle studies showing decreased feed intake on an E+ diet under different environmental conditions (Hemken et al., 1979Go, 1981Go; Hoveland et al., 1980Go; Schmidt et al., 1982Go; Jackson et al., 1984aGo,bGo; Osborn, 1988Go; Aldrich et al., 1989Go, 1993Go; Stuedemann et al., 1989Go; Osborn et al., 1992Go). Partial recovery of feed intake did occur after approximately 1 wk on the E+ diet. A similar recovery was noted by Roberts et al. (2002)Go over the first 4 d at thermoneutrality before exposure to heat. The reason for the large, rapid decrease in feed intake is unknown. Because the E– rats continued eating the control diet, it was not due to the amount of seed in the diet. Moreover, it is unlikely that palatability of the ground seed diet was responsible for the decreased feed intake because the E– diet, containing the identical content of fescue seed, resulted in no change in feed intake. Possible explanations that remain to be tested are related physiological change and include suppression of appetite following intake of infected tall fescue seed. In addition, studies of longer duration are needed to determine whether the partial recovery of feed intake is due to adaptation.

As expected, BW and growth rate paralleled the decrease in feed intake during the preheat period. The decrease in growth rate was delayed behind the decrease in feed intake, with the first significant group difference in BW (P < 0.05) occurring on d 3 (Figure 1Go). Rate of increase in BW and ADG were less for E+ than for E– rats during this period by 70 to 78%. The decrease in ADG in the present study (78%) approximates that of Neal and Schmidt (1985)Go for rats fed an E+ seed diet under similar conditions (63%). Likewise, it approximates the decrease in ADG for cattle fed E+ under a variety of ambient conditions, where the decrease ranged from 22 to 79% (Schmidt et al., 1982Go; Stuedemann et al., 1986Go).

Rats in the present study exhibited a prominent circadian rhythm for Tc even when maintained at a constant environmental temperature. The noted increase in Tc at night and decrease during the day (Figure 2Go) is typical of nocturnal rodents as reported by others (Scales and Kluger, 1987Go; Gordon, 1993Go).

A reduction in Tc of E+ rats occurred from d 3 to 5, which remained in effect for the duration of the preheat period (Figure 3Go). A similar change did not occur in the E– group. There are no reported studies of a reduction in internal temperature of any animal following long-term intake of an E+ diet under controlled conditions. However, several studies have shown that intraperitoneal injection of ergovaline in the rat produces an acute decrease in core temperature under thermoneutral and cold stress conditions (Zhang et al., 1994Go; Spiers et al., 1995Go). This decrease may be due in part to a reduction in metabolic heat production. It remains to be determined whether long-term intake of E+ toxins produces a similar physiological response under similar thermal conditions.

The largest decrease in core temperature occurred in the minimum daily value, as shown for d 8 (Figure 2Go) with onset of the light period. General activity also was least during the light period (Figure 2Go), and was slightly less for the E+ than for the E– rats. This lowered activity might have contributed to the reduced core temperature at this time of day. There is a correlation between activity and core temperature in the rat, as reported by Gordon (1993)Go, who noted that the increase in activity during the nocturnal phase of the daily cycle is associated with an increase in internal temperature. It is possible that the reduction in activity of E+ rats was due to the added effects of stress and general sickness behavior. Meerlo et al. (1996)Go observed that stress in rats produces a decrease in activity during the night phase. Likewise, fever in rats produces a decrease in activity (Luker et al., 2000Go) that is characteristic of sickness behavior. It is possible that intake of E+ toxins in the rat produces a malaise that is characterized by a general decrease in activity, and unrelated to direct effect on specific physiological systems.

Heat Stress

Exposure to heat stress resulted in a reduction in feed intake of rats in both treatment groups. Overall reductions in feed intake from preheat to heat periods for E– and E+ groups were 31.7 and 21.5%, respectively. Others have shown feed intake in the rat is decreased with an increase in body temperature (Di Bella et al., 1981aGo,bGo) and exposure to heat stress (Johnson and Cabanac, 1982Go; Johnson and Strack, 1989Go; De Vries et al., 1993Go). As in the present study, there is a reduction of 15 to 40% in feed intake during heat stress in rats (Johnson and Cabanac, 1982Go). This reduction is due to decreases in total feeding time, duration of each feed, and frequency of feed (Johnson and Cabanac, 1982Go; Johnson and Strack, 1989Go).

It seemed in the present study that most of the decrease in feed intake attributable to the E+ diet occurred before heat stress, with the effect of heat stress on E+ and E– groups being similar for this variable. A similar heat-induced decrease in feed intake of E+ and E– rats was noted by Roberts et al. (2002)Go. It is often reported that large domestic animals exhibit decreased feed intake when fed an E+ diet in the heat. Aldrich et al. (1993)Go noted a greater decrease in feed intake by cattle fed an E+ diet at 32°C than at 22°C. Others have reported that cattle on an E+ diet exhibited a feed intake reduction of 57 to 60% below E–level at air temperature above 31°C (Hemken et al., 1981Go; Osborn, 1988Go; Osborn et al., 1992Go). Lambs maintained under constant heat stress (33°C) and fed an E+ diet decreased feed intake by 28% below E– level (Gadberry et al., 2003Go). The results of the present rat study agree with the findings for large domestic animals, in that there is a greater decrease in feed intake of E+ animals than E–, and there is further depression of intake during heat stress. It is possible that heat stress simply decreases the intake of both groups by the same percent, with the E+ animals remaining at the lowest level.

Exposure to heat stress in the present study decreased the ADG of both treatment groups (Figure 1Go). As noted earlier, this decrease was expected given the decrease in feed intake. Others also have reported heat stress reduces BW and growth rate in rats (Ray et al., 1968Go; Johnson and Strack, 1989Go). The combination of heat stress and E+ treatment reduced overall ADG from thermoneutral level by 35%. Similar results were noted by Roberts et al. (2002)Go. Percent reduction in ADG of E+ rats in the present study below E– level in the heat was 63%. Lambs exposed to continuous heat of 33°C and fed an E+ diet exhibited a significant decrease in weight gain compared with lambs fed an E–diet (Gadberry et al., 2003Go).

Many studies of cattle and sheep have shown that intake of an E+ diet during heat stress in field or laboratory environments results in hyperthermia above control level (Hemken et al., 1979Go; 1981Go; Hoveland et al., 1983Go; Bond et al., 1984Go; Jackson et al., 1984aGo,bGo; Goetsch et al., 1987Go; Strahan et al., 1987Go; Osborn, 1988Go; Rhodes et al., 1991Go; Osborn et al., 1992Go; Al-Haidary et al., 2001Go; Gadberry et al., 2003Go). Although it was shown in an earlier study that injection of ergovaline during heat exposure (i.e., 31 to 33°C) results in hyperthermia (Spiers et al., 1995Go), there has been no previous study using rats that has shown a similar hyperthermia during long-term intake of E+ seed under heat stress conditions; rats in the present study exhibited similar responses to dietary intake of E+. This response was short-lived reaching a peak on d 14, with a gradual decrease thereafter (Figure 3Go). Rats are known to adapt to heat stress by decreasing metabolic heat production (Rousset et al., 1984Go; Arieli and Chinet, 1986Go; Shido et al., 1991Go; Gordon, 1993Go) and increasing heat loss activities (Wyndham, 1967Go; Horowitz et al., 1983Go). Although not measured in the present study, it is likely that either or both avenue(s) contributed to the adaptive temperature change of both treatment groups from d 14 to 30.

There have been no quantitative studies of the effect of E+ toxins on animal activity in the heat. In the present study, there was a decrease in rat activity in the heat that remained in effect for the duration of the heat stress period (Figure 3Go). The decreased activity was likely due to a decrease in heat production in an attempt by the animal to limit the increase in core temperature. Others have noted that rat activity is reduced during heat exposure and observed a concomitant decrease in metabolic heat production during this time (Shido et al., 1991Go; Gordon, 1993Go). Rats on a E+ diet exhibited a lower activity level than those on a E– diet. This difference increased after approximately one week in the heat, when the E+ rats reached their nadir in activity. This lowered activity remained in effect for the remainder of the stress period, with no indication of recovery. The present study shows that fescue toxicosis results in an even greater decrease in activity during heat stress, and warrants a study of large domestic animals to determine whether there is a similar effect.

Postheat

To our knowledge, there have been no studies of recovery in performance from the combined effects of heat stress and intake of E+ toxins. In the present study, recovery from heat stress was measured while the animals remained on E+ and E– treatment diets to determine whether there was a return to the original preheat condition or adaptive change. Both treatment groups exhibited an immediate parallel increase in feed intake upon removal from the heat stress environment (Figure 1Go), but retained preheat differences. There was no evidence of adaptation in the feed intake response from preheat level. Growth rate also increased following removal from heat, likely as a result of the improved feed intake. These results suggest that the return of feed intake to a normal level following heat stress may be as important as the onset of response.

Return of rats to thermoneutral condition in the postheat period resulted in a return of core temperature to preheat level, with E+ rats exhibiting a lower temperature than E– rats (Figure 3Go). In fact, the core temperature of E+ rats continued to drop approximately 0.3°C below preheat level to suggest a decreased ability to maintain homeothermy under thermoneutral conditions, whereas the E– rats remained stable at preheat level. Group differences in core temperature were evident even on the last day of the study (d 37), when daily minimum Tc for the E+ group was 0.42°C below the E– group.

The results of this study show that intake of E+ diet at thermoneutrality decreased core temperature, which is consistent with the hypothermia noted by Neal and Schmidt (1985)Go under similar environmental conditions. We now know that this decrease can occur even after exposure to heat stress. One possible cause for the decrease in temperature is a reduction in metabolic heat production. Earlier studies have shown that intraperitoneal injection of rats with ergovaline results in a rapid decrease in metabolic heat production under thermoneutral (Zhang et al., 1994Go) and cold stress (Spiers et al., 1995Go) conditions. A similar reduction in heat production could have occurred with chronic intake of E+ diet in the present study.

Activity also changed during the postheat period, with a rebound followed by a return to preheat level (Figure 3Go). The large rebound in activity following removal from the heat was unexpected and has not been previously reported in association with fescue toxicosis. During this period, the activity of the E+ rats remained significantly below that of the E– rats. The decreased level might result in lowered heat production and contribute to the reduced core temperature of the E+ group. Likewise, the activity level of E+ rats on d 37 (the last day of the study) is below the preheat level, and corresponds to the differences in core temperature for these periods. It is unknown when or whether the activity of E+ animals would return to preheat level.

The results of this study show that intake of E+ produces symptoms in rats that are similar to those associated with fescue toxicosis in cattle, and suggest that the rat model is appropriate for the study of this condition. In addition, this study extends the known symptoms of fescue toxicosis to include a decrease in general activity. More importantly, this controlled, long-term study shows that several of the common animal endpoints used in the study of fescue toxicosis are highly dependent on ambient temperature. A change in ambient temperature from thermoneutral to one producing heat stress condition produces significantly different responses in terms of feed intake, growth rate, core temperature, and activity. In fact, the body temperature response is actually reversed with a shift in environments. Such changes are typical during the summer period in many regions of the United States. This suggests that there may be large variations in these responses over time in field studies, which can only be understood with detailed recording of variables throughout the test period.


   Implications
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Implications
Literature Cited
 
Rats fed a diet containing endophyte-infected fescue seed for 37 d exhibited symptoms characteristic of fescue toxicosis in both thermoneutral and heat stress environments. Feed intake and growth rate were decreased below control level in both environments. Core body temperature was elevated above control level primarily during the first week of heat exposure, with recovery thereafter. Activity of treated rats was depressed throughout heat exposure, with no sign of recovery in this environment. The rat provides a cost-effective and reliable model for fescue toxicosis that will allow for evaluation of potential treatments and mechanism(s) of action under chronic conditions. Likewise, this model can be used to identify markers of susceptibility to fescue toxicosis that will ultimately be tested in large domestic animals.


   Footnotes
 
1 Animal use in this study was approved by the Univ. of Missouri-Columbia Animal Care Committee. Back

2 The authors would like to thank K. Arevalo, L. Butcher, II, and T. Lund for data collection and initial analysis of the results. In addition, we would like to acknowledge the assistance of K. Fritsche (Anim. Sci. Dept., Univ. of Missouri) in developing the rat diets used in this study. Partial funding for this study was received from the Missouri Agric. Exp. Stn. In addition, this material is based on work supported by the USDA, under Agreement No. 6227-31230-004-I5S. Any opinions, findings, conclusion, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the USDA. Back

3 Correspondence: 114A Animal Science Research Center (phone: 573-882-6131; fax: 573-882-6827; e-mail: spiersd{at}missouri.edu).

Received for publication September 16, 2004. Accepted for publication January 14, 2005.


   Literature Cited
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Implications
 Literature Cited
 


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